I. Field of the Invention
The present invention relates to a cassette used for shipping and transporting various items. More specifically, the invention relates to cassettes specifically designed for holding wafers, rigid memory disks, flat panel displays, liquid crystal displays, or photomasks. The invention is particularly advantageous when used to hold relatively large items such as wafers having, for example, a diameter of 300 mm or more. However, cassettes made in accordance with the invention can also be used to hold smaller items.
II. Description of the Prior Art
Various containers have been used in the prior art to hold silicon wafers, rigid memory disks, flat panel displays, liquid crystal displays and the like. Such items must be protected as they are transported from point to point within a manufacturing facility because they tend to be very fragile. Loading such items into cassettes for transport has proven to be an effective means of providing protection.
Early cassettes were heavy, bulky and expensive. These early cassettes also were not well adapted to the environment in which they had to function. The heat and harsh chemicals used in such environments often weakened such prior art cassettes causing them to bend, flex, warp or even break during processing. Such damage to the cassette would often destroy the items stored in the cassette.
Later cassette designs sought to overcome the disadvantages of the prior art by providing various gussets, supports or H-bars to reinforce the cassette to prevent such warpage. Also, materials such as Teflon.RTM. were used. These materials were selected to withstand chemical etching processes to which the items are routinely subjected. Such techniques were adequate for cassettes designed to hold smaller items such as those having a length or diameter of approximately 150 mm or less. However, given recent trends in the semiconductor and electronics industry, new cassette designs are needed which can accommodate items of a larger size. Such cassettes must be designed so that they will not flex, warp, bend or break, even when subjected to high temperatures or harsh chemicals. Further, given the size and weight of such items, robotic handling becomes more and more important thereby requiring structures on the cassette which can assist in indexing and positioning the cassette with respect to processing equipment.